1. Field of the Invention
The present invention relates to a method for forming metal wiring structures that can be used favorably in producing fine semiconductor devices.
2. Description of the Related Art
In the formation of wiring layers in semiconductor devices, insulation films of low dielectric constant are used as insulation layers between wires, while Cu and other metal wires of low resistance are used for wires, in order to prevent delays in signal transmission. When forming a Cu wiring structure, a general method is to form an interlayer insulation film on the Cu wiring in the bottom layer, and then form in this insulation film a lower layer and trenches that become a wiring pattern in an upper layer and contact vias that connect the upper layer, to create a damascene structure. In this case, formation of a Cu film, which becomes the wiring pattern in the top layer, directly on the insulation film will cause Cu to diffuse in the insulation layer and affect the device characteristics. Accordingly, a metal barrier layer is formed between the Cu film and the insulation layer to prevent diffusion of Cu.
This metal barrier film is formed by various methods such as the sputter method, CVD method and ALD. Materials used to form barrier films include TiN, TaN and WN. As semiconductor devices become increasingly fine, CVD films offering good step-coverage have been adopted as barrier films, and ALD films have been adopted more recently. Materials used to form these films include TiCl4, TaF5, WF6 and other halide compounds, while NH3 and other gases are used to form nitrogen films. Compared with PVD, CVD and ALD make the interlayer film more vulnerable to damage because of the chemical reaction with the material gases. In many cases, CVD and ALD do not provide sufficient adhesion. Furthermore, barrier films formed per CVD or ALD do not necessarily offer good adhesion with Cu film. For these reasons, insertion of Ta or Ru between the CVD or ALD-formed barrier film and Cu has been proposed as a means to improve adhesion.
International Publication No. WO03/056612 A1 discloses a structure in which a Ru film is formed on a barrier layer film by means of plasma ALD, as well as a structure in which a Ru film is formed directly on an interlayer film. Here, a Cu film is formed on the Ru film by means of CVD or electroplating.
U.S. Pat. No. 6,759,325 B2 discloses a method to prevent an ALD-formed barrier film from diffusing into a SiOC film through pores when a metal barrier is formed using ALD on a porous SiOC film, provided as an interlayer insulation film, which has a lower specific dielectric constant than a SiO2 film. This method provides, among others, for a method to use PVD or CVD to form a conductive film over the surface of an interlayer insulation film to seal the pores near the surface layer, and then form a metal barrier using ALD.
JAP Vol. 95, Number 1, pp. 381-388 (2004) discloses a method to form an ALD-WNC film. To be specific, an ICP plasma apparatus or TCP plasma apparatus is used to plasma-process a low dielectric constant surface with a mixture gas containing oxygen and nitrogen, after which WF6, TEB and NH3 gases are introduced one by one. When this process is repeated, a smooth WNC film can be formed while preventing tungsten from diffusing into the low dielectric constant film. Here, it is claimed that existence of oxygen and nitrogen on the surface is effective in forming a uniform WNC film. The oxygen content is in a range of 99 to 20%. The insulation film of low dielectric constant, which is constituted by SiOC, is a porous film and its pore size changes depending upon the forming method. It is reported that the aforementioned plasma process prevents the formed WNC film from permeating into the insulation film.
In the formation of a capacitor using a Ru film as an electrode, smoothness of the Ru film is very important. In the case of a capacitor, an extremely thin dielectric film must be formed on an electrode. If the Ru film has a very rough surface, the electrical characteristics of the dielectric film deteriorate easily. Publication of Unexamined Patent Application No. 2003-168738 discloses a method to form a metal barrier directly under a Ru film as a bottom electrode, when forming a capacitor.